Traction hoists move up and down a wire rope (cable) by creating friction between the wire rope and one or more sheaves and drums. Current traction hoists are exemplified by U.S. Pat. Nos. 4,139,178 and 3,944,185. These hoists include a motor, power transmission, cable driving sheave, cable tensioning sheave, a pressure exerting chain, and a number of brake mechanisms. These major components are all mounted to the same framework--a large casting, with the drive sheave radially stationary and the chain moving to bias the cable.
While the prior art hoists generally operate in a satisfactory manner, the mechanisms involved are heavy, complex, and require a relatively large number of moving parts. As a result, such devices are expensive and require significant service and maintenance. Reeving of the cable through many of the prior art hoists is a time consuming process, since it may involve passing an extensive amount of cable through the hoist. The cable is subject to excess wear by unnecessarily large forces pressing the cable against the drive sheave. Forces on the drive sheave, by virtue of its mounting to the overall framework, can be transmitted to the gearbox and gearbox shaft. This is particularly of concern in an overload (overhung) situation, where damage to these expensive parts could result (requiring as a precaution much heavier gearboxes and shafts). While the prior art devices do employ safety brakes, they typically work as a function of velocity of cable movement, i.e., overspeed brakes. However, it is also desirable to automatically brake the system in overload and underload situations.